Suppressor cell activity is high in the spleen of neonatal mice. To test the hypothesis that high suppressor cell activity, in newborn mice, is (in part) responsible for immunological immaturity, we propose to: Determine if there is a correlation between the decrease in suppressor cell activity and the development of cell-mediated immunity during postnatal development. Establish whether removal of suppressor cells from the spleen of newborn mice will allow for the expression of cell-mediated immunity. Study the mechanism by which suppressor cell activity declines after birth. In addition we propose to compare the nature of the suppressor cell in the neonatal mouse with the nature of the suppressor cell which develops in the adult mouse after tumor transplantation.